In recent years, semiconductor lasers have been used in various equipment including optical systems since the energy efficiency of a semiconductor laser is high.
Semiconductor lasers often are subject to undesirable variation in the wavelength and output power because of fluctuations in the operating temperature of the semiconductor laser.
Also, the wavelength and output power fluctuate because of variations in the injection current of a current supply source. (For example, see a search report under the title of "Frequency and Power Stabilizations of GaAlAs Diode Laser on an Opto galvanic Effect" of Shingaku Giho Vol. 82. No. 218, OQE-99 of Technical Search Report of Denshitsushin Gakkai; OQE82-95 to 106 (issued on Jan. 17, 1983)).
The present applicant has filed an application (see Japanese Patent Early Laid-open Publication No. Sho 62-244184, Japanese Patent Application No. Sho 61-87349 filed on Apr. 16, 1986) regarding an apparatus for stabilizing wavelength and output power of a semiconductor laser. The apparatus described in this Japanese Patent Early Laid-open Publication Sho 62-244184 is designed such that injection current is adequately controlled to stabilize the wavelength. The injection current is controlled based on light received through an interference filter. The interference filter has varying spectral characteristics in wavelength range of semiconductor coherent light emission. For example, the interference filter may increase in transmittance with an increase in wavelength, or vice-versa. Additionally, the operating temperature is controlled to stabilize the output power of the semiconductor laser.
The apparatus described in this Japanese Patent Early Laid-open Publication No. Sho 62-244184 can stabilize the wavelength and output power of the semiconductor laser to some extent. However, when a highly accurate measurement is required, for example in a gage interferometer, etc., a higher degree of stabilization is demanded.
In the apparatus described in the above-mentioned Japanese Patent Early Laid-open Publication No. Sho 62-244184, the stabilization of wavelength and output power depends upon the spectral characteristics of the interference filter. Also, the stabilization depends upon variations in a reference voltage used as a reference for detecting wavelength fluctuation. The reference voltage is compared with a signal showing the wavelength variation of the light which has passed through the interference filter. The reason why the degree of the amplitude change with wavelength in the spectral characteristics of the interference filter contributes to the stabilization of the wavelength is that the control signal controls the injection current, to which the wavelength quickly responds.
If transmissivity of the interference filter changes rapidly with a small change in wavelength, selected small changes in wavelength can be more easily detected. However, this narrows the range of wavelengths which may be detected and controlled by a given interference filter.
Also, the reference voltage may fluctuate over time from various known causes, such as aging of components.
Furthermore, the signal which represents the wavelength fluctuation of the laser light fluctuation of the output power of the laser. Therefore, the feed back system compares the reference voltage with the signal which represents the wavelength fluctuation. This feed back system has the shortcoming that it cannot correct wavelength variations caused by the fluctuation of the output power of the semiconductor laser.